The eye relief of an optical instrument (such as a telescope, a microscope, or binoculars) is the distance from the last surface of an eyepiece within which the user's eye can obtain the full viewing angle. If a viewer's eye is outside this distance, a reduced field of view will be obtained.

Eye relief (ER) is the distance your eye must be from the telescope eyepiece lens to achieve focus. Long eye relief is of importance to eyeglass wearers, who will likely prefer longer lengths to be comfortable, such as something over 15mm. This is a matter of comfort, however, and that comfort le...